Recirculation of a controlled amount of diesel engine exhaust gas to the engine air intake is generally known to provide a beneficial reduction in diesel engine emissions of oxides of nitrogen NOx. Limits are conventionally imposed on the amount of recirculated exhaust gas EGR to avoid excessive engine intake air charge dilution which may degrade engine performance and increase particulate emission levels.
Both open-loop and closed-loop EGR control approaches operate to deliver EGR to the engine air intake in an amount reflecting a compromise between the competing goals of minimizing NOx and particulate emissions in an engine that delivers a satisfying level of performance. The open-loop approaches deliver EGR according to an open-loop EGR schedule determined through a modelled or calibrated relationship between desired EGR and certain engine parameters. Such open-loop approaches are sensitive to variations in that modelled or calibrated relationship, such as may result from sensor or actuator degradation over time, or from gradual changes in such parameters as engine exhaust backpressure.
Closed-loop approaches attempt to compensate for unmodelled or unmodellable system disturbances by including some measure of the actual performance of the EGR control in the determination of a desired EGR quantity. For example, the approach described in U.S. Pat. No. 5,150,694, assigned to the assignee of this invention, relies on actual engine air/fuel ratio feedback from a conventional oxygen sensor in the diesel engine exhaust gas path to compensate for deviations in the actual performance of the EGR control away from a desired performance. Sensitivity to unmodelled system disturbances is reduced through such closed-loop operation, but at a cost. The oxygen sensors and their supporting electronic circuitry add significant expense to the EGR control. Additionally, sensor contamination and sensor exposure to temperature extremes in the harsh exhaust gas path environment can lead to reduced sensor accuracy, which can result in reduced EGR control precision.
Accordingly, it would be desirable to realize EGR control sensitivity reductions through a closed-loop EGR control approach that does not add significant cost to the control system. Further, it would be desirable to rely on feedback from sensors not subject to the harsh environment of the engine exhaust gas path, so that control precision loss resulting from sensor contamination and sensor exposure to temperature extremes may be reduced.